Clinical examination and diagnosis of diseases are performed by detecting genes and proteins related to the diseases which are contained in biological samples by detection methods such as a gene detection method and an immunological detection method. Examples of the detection methods include immunochromatography, latex agglutination, enzyme immunoassay, chemiluminescent immunoassay, and PCR assay.
On the other hand, a method of using an electric current generated by photoexcitation of a photochemically active labeling substance and light or an electric current generated by applying a voltage to an electrochemically active labeling substance to detect analytes such as genes and proteins is proposed for the purpose of improving detection sensitivity, quantitative performance, and simplicity (see, for example, U.S. Patent Publication No. 2009/294305, U.S. Pat. No. 5,776,672, and U.S. Pat. No. 5,972,692).
U.S. Patent Publication No. 2009/294305 describes a method of detecting an analyte, comprising: irradiating the analyte labeled with a photochemically active sensitizing dye with light; and measuring an electric current caused by photoexcitation of the sensitizing dye contained in the labeled analyte (hereinafter referred to as “photoelectrochemical detection”). In the method described in U.S. Patent Publication No. 2009/294305, the labeled analyte is brought into contact with a working electrode containing a trapping substance capable of binding directly or indirectly to the labeled analyte on the surface. Thus, the labeled analyte is immobilized on the working electrode through the trapping substance. Subsequently, the working electrode and a counter electrode are brought into contact with an electrolyte medium, and the labeled analyte immobilized on the working electrode is irradiated with light to excite the sensitizing dye. Thereafter, the analyte is specifically detected by measuring a photocurrent which flows between the working electrode and the counter electrode due to electronic transition from the photoexcited sensitizing dye to the working electrode.
U.S. Pat. No. 5,776,672 and U.S. Pat. No. 5,972,692 disclose gene detection methods using an electrode on which a single-stranded nucleic acid probe having a base sequence complementary to the gene to be detected is immobilized and a double-strand recognizing substance which specifically binds to double strand nucleic acid and contains a labeling substance which is electrochemically active. In the methods described in U.S. Pat. No. 5,776,672 and U.S. Pat. No. 5,972,692, a sample containing nucleic acid that is denatured into a single strand, a probe, the double-strand recognizing substance are contacted with one another. Then, a target gene is detected by measuring an oxidation reduction current and electrochemical luminescence based on the labeling substance contained in the double-strand recognizing substance bound to a double strand nucleic acid which is formed by hybridization between a nucleic acid corresponding to the target gene and a probe.
In these detection methods, the analyte is detected through a labeling substance that is electrochemically or photochemically active. Thus, the working electrode in which a trapping substance for trapping the analyte is immobilized on the surface so that the labeling substance is present near the working electrode depending on the amount of the analyte is used.
The working electrode in which a trapping substance is immobilized is hard to be reused. This is because, in order to reuse the working electrode with the trapping substance immobilized, it is necessary to remove substances other than the trapping substance on the working electrode by a cleaning process. However, the trapping substance may also be removed from the working electrode in the cleaning process. Further, the trapping substance on the working electrode may be denatured by a cleaning agent to be used in the cleaning process. Thus, the time of reusing the working electrode with the trapping substance immobilized may affect the measurement results. Therefore, a detection unit which includes the working electrode with the trapping substance immobilized is usually thrown away for each measurement process. Consequently, a detection system using the working electrode with the trapping substance immobilized has a disadvantage of higher costs per measurement.
As for the detection system using the working electrode with the trapping substance immobilized, when the analyte is large, it becomes difficult to allow the labeling substance to be present near the working electrode due to steric hindrance, resulting in reduced detection performance.